The present invention relates to a system and method of print media back-feed control for a printer, and more particularly to a system and method incorporating a downstream sensor for assisting controlled back-feeding of the print media.
In order for a printer to accurately and repeatedly print a defined image (e.g., text, graphics, etc.) on a particular print media (e.g., paper, adhesive label, transparency, etc.) the spatial relationship between the printer and the print media must be sensed, programmed, or otherwise known by the printer. Moreover, the parameters of the print media (generally including the bounds of the print area of the particular print media) must also be available to the printer. In most printing applications, the print media typically defines a leading edge, a top (i.e., generally the first available print area of the print media as the print media travels through the printer during printing), and a print line location (i.e., the location on the print media where the print head is programmed to print at a particular moment in time). As a result, defined spatial relations, both between the print media and the print head, and relative to the bounds of the print media, allow a printer to produce the desired result.
The spatial relation between the printer and the print media is more important in certain printing applications. For example, a thermal transfer printer incorporates print media that is often packaged as a rolled web of individual repeating labels. The print media is unrolled and directed along a path that passes between the print head (i.e., the general structure that imparts the image to the print media) and an adjacent platen roller. The force established between the print head and the platen roller is referred to as the nip pressure. The nip pressure is designed to create sufficient friction between the print media and the platen roller to allow the platen roller to direct the print media downstream or upstream relative to the print head. Proper operation of the printer requires that a portion of the print media remain between the print head and platen roller at all times, thereby ensuring that the nip pressure maintains the platen roller in driving engagement with the print media.
Programming or manually setting the spatial relations and print media parameters for each print media is repetitive and time consuming. As a result, printers often include a sensor positioned between the print media supply spool and the print head to detect at least the start of a label or other print media. Thus, as the print media is directed toward the print head, the sensor senses indices (e.g., notches) of the print media that are correlated to a particular print line location, such as the top of the print media of a preprogrammed print media having known parameters. In effect, the spatial relationship between the print media and the print head of the printer is therefore defined.
The above techniques work well until the spatial relationship between the print media and the print head is degraded or becomes undefined. This may occur when the power to the printer is cycled, when a new web of print media is installed, as the result of a print media jam, or many other circumstances that prevent a printer controller from knowing the present spatial position of the print media relative to the print head.
The spatial relation between the print media and the print head can be reestablished in a variety of ways. For example, the printer can simply feed the print media downstream until the sensor between the print media and the print head senses the top of the print media; however, this approach results in wasted print media. Alternatively, the print media may include complex indices that provide a relatively accurate indication of the present location of the print media; however, this results in expensive print media due to the addition of the detailed indexing and may not provide the needed accuracy for high-quality printing.
An additional approach is to back-feed the print media upstream toward the print head; however, this results in the print media being inadvertently back-fed beyond the print head, resulting in a loss of nip pressure acting on the print media at the interface between the print head and the platen roller. Without the nip pressure, the platen roller is unable to drive the print media requiring user intervention to re-feed the print media between the print head and the platen roller—wasting the user's time and causing frustration.
In light of the above, a need exists for a system and method that provide for efficient, economical, and autonomous print media back-feed control.